Transferring large data objects across a data communications link is a problem for multimedia/imaging applications utilized in client/server distributed computing systems (such as the Internet's World Wide Web). One attempt at solving this problem is disclosed in U.S. Pat. No. 4,897,781, Jan. 30, 1990, titled "System And Method For Using Cached Data At A Local Node After Re-Opening A File At A Remote Node In A Distributed Networking Environment." In this system, the client processing system employs a client cache within its operating system to store data files. Utilizing the client cache minimizes the number of reads and writes that must be effected over the network to the server processing system where the file physically resides. This system prevents a process in the client processing system from accessing data in the client cache that has been modified at another node in the network by testing blocks of data in the client cache for "validity" by using modification times measured by the server processing system. In this approach, a cached file is considered invalid if its time stamp is different from the time stamp of the corresponding server file.
Another approach is disclosed in U.S. Pat. No. 5,493,728, Feb. 20, 1996, titled "System And Methods For Optimized Access In A Multi-User Environment." In this system, what is sometimes called a "dirty bit" is set periodically in the server file, and the client operates on a copy residing in a local rapid-access memory until the server file is again marked as being invalid.
One shortcoming inherent in the prior art is that the decision as to whether the cached copy of a requested data object is "valid" is made on the basis of factors not directly related to the content of the data object itself, which could lead to inefficient operation of the system in certain situations. For example, identical data objects could have different timestamps, and thus the approach disclosed in the '781 patent could result in another copy of a requested object being sent to the client even though the locally-cached object is perfectly valid in that it is the same as the current object retained by the server. Accordingly, a primary goal of the present invention is to provide an improved system for operating a distributed network wherein large data objects must be transmitted across a communications link of limited bandwidth.